This invention relates to plastically deformable mixtures based primarily on well dispersed sinterable metallic particles, organic binding agent, and polar liquid. The invention provides homogeneous mixtures and processes for forming or shaping them into useful articles or structures. Particularly useful shapes formed by the practice of this invention are honeycomb-type articles or honeycombs which may be used for numerous applications including filters, catalyst supports, and heaters.
The need for uniform dispersion in deformable mixtures has long been recognized in the art, particularly in the formation of complex articles. Non-uniform dispersion leads to variability within the mixture which can lead to process variability as well as inconsistencies in final product characteristics. Non-uniform dispersion is often caused by the use of immiscible solvents or components in the mixtures which leads to the formation of localized areas of plastic and non-plastic mixtures during the mixing process. This lack of uniformity often leads to other problems and difficulties both in the forming of such mixtures and in the quality of the formed articles or structures. It is generally known that processing of non-uniformly dispersed mixtures consumes considerable power and require high extrusion pressure. Extrudates of such mixtures tend to be dry and non-cohesive. The extrudates tend to exhibit localized wet areas which may lead to uneven drying and distortions or collapse of the formed articles. Finally, such mixtures tend to damage the mixing equipment, particularly the mixing contact mechanisms as well as the drive motors of such equipment.
U.S. Pat. No. 4,992,233 issued to Swaroop et al. describes the formation of monolithic metal honeycombs from a mixture or batch including metal particles, an organic binder or binding agent, water, and a long chain aliphatic acid useful as a wetting agent, and a metal soap useful as a lubricant. Generally, long chain aliphatic acids are immiscible in polar liquids such as water. As a result, the mixture is non-uniformly dispersed and is subject to the problems discussed above.
There have been attempts made to solve the problems associated with differential shrinkage caused by uneven drying or liquid removal. Differential shrinkage leads to cracking, localized distortion, or collapse of the article, among other problems. U.S. Pat. No. 4,965,039 issued to Schuetz describes a method of making an inorganic slurry, forming the slurry into a flexible article, and rapidly removing the solvent medium from the article.
Problems, such as cracks, gas generation, fissures, bubbles and other aesthetic and structural defects in finished articles have also been noted in the ceramic forming industry. U.S. Pat. No. 4,996,015 issued to Yoshimoto et al. describes a method of forming ceramic honeycomb structures by the careful removal of a water/organic solvent system to avoid such structural and surface defects.
There is no teaching in these references of the use of water-miscible surfactants and/or solvents to provide a homogeneous, uniformly dispersed stable mixture free of bubbles or frothing. It is therefore the object of this invention to introduce into such mixtures a component which is compatible with polar liquids, to allow for uniform dispersion, and yield a homogeneous, bubble-free or stable plastically deformable mixture.